Is there someone can explain this to me how?

How do they do this?
I found a tvsite and when i click the watch now button of my selected tv show, same page loads but with an addition, when the page loads it adds/inserts the video and the content gateway appears. Also, i tried to view its source and found out that the watch now button is linked to ?a=watchnow which adds at the end of the permalink when clicked.

the page is like hxxp://sampletvsite.xxx/show-season1
when the watch now button is clicked, it directs to this link
hxxp://sampletvsite.xxx/show-season1?a=watchnow where the video and dollarade content gateway appears

can someone explain this to me how they can insert video and appear the content gateway by just adding ?a=watchnow at the end of the permalink?

use some plugin like easy-wordpress-content-locker if you are using wordpress ..

Plug-ins are the first option other than use gadgets to insert watch now button